1. Field of the Invention
The present invention relates to a heat treatment method of a thermoplastic resin film and an apparatus thereof, and particularly a heat treatment method of a thermoplastic resin film such as a cellulose acylate film used as a retardation film in a liquid crystal display device, and an apparatus thereof.
2. Description of the Related Art
Uses of a cellulose acylate film have been enlarged as an optical film for an image display device, typically, such as a liquid crystal display device and an organic EL display device in view of transparency, rigidity and optical isotropy thereof. An optical film for a liquid crystal display device is used as a retardation film by stretching the film to exhibit in-plane retardation (Re) and thickness direction retardation (Rth), other than as a protective film for a polarizing plate.
In a production process of a thermoplastic resin film such as a cellulose acylate film, there may be a case where a heat treatment is carried out while holding the both edges of the film in a width direction. For example, in case of film formation by a solution film formation method, in a drying step for volatilizing a solvent, a film is dried by heating while holding the both edges of the film in a width direction. When a film is transversely stretched in a width direction, the film is stretched in a width direction at a specified heating temperature while holding the both edges of the film in the width direction. Further, when a film is crystallized, crystallization is promoted by heating while holding the both edges of the film in the width direction (for example, see Japanese Patent Application Laid-Open Nos. 6-166089 and 8-244051).